二叉树遍历（前序、中序、后序）-Java实现

一、前序遍历

　　访问顺序：先根节点，再左子树，最后右子树；上图的访问结果为：GDAFEMHZ。

　　1）递归实现

    public void preOrderTraverse1(TreeNode root) {
        if (root != null) {
            System.out.print(root.val + "->");
            preOrderTraverse1(root.left);
            preOrderTraverse1(root.right);
        }
    }

　　2）非递归实现

    public void preOrderTraverse2(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        while (node != null || !stack.empty()) {
            if (node != null) {
                System.out.print(node.val + "->");
                stack.push(node);
                node = node.left;
            } else {
                TreeNode tem = stack.pop();
                node = tem.right;
            }
        }
    }

二、中序遍历

　　访问顺序：先左子树，再根节点，最后右子树；上图的访问结果为：ADEFGHMZ。

　　1）递归实现

    public void inOrderTraverse(TreeNode root) {
        if (root != null) {
            inOrderTraverse(root.left);
            System.out.print(root.val + "->");
            inOrderTraverse(root.right);
        }
    }

　　2）非递归实现

    public void inOrderTraverse(TreeNode root) {
        Stack<TreeNode> stack = new Stack<>();
        TreeNode node = root;
        while (node != null || !stack.isEmpty()) {
            if (node != null) {
                stack.push(node);
                node = node.left;
            } else {
                TreeNode tem = stack.pop();
                System.out.print(tem.val + "->");
                node = tem.right;
            }
        }
    }

三、后序遍历

　　访问顺序：先左子树，再右子树，最后根节点，上图的访问结果为：AEFDHZMG。

　　1）递归实现

    public void postOrderTraverse(TreeNode root) {
        if (root != null) {
            postOrderTraverse(root.left);
            postOrderTraverse(root.right);
            System.out.print(root.val + "->");
        }
    }

　　2）非递归实现

    public void postOrderTraverse(TreeNode root) {
        TreeNode cur, pre = null;

        Stack<TreeNode> stack = new Stack<>();
        stack.push(root);

        while (!stack.empty()) {
            cur = stack.peek();
            if ((cur.left == null && cur.right == null) || (pre != null && (pre == cur.left || pre == cur.right))) {
                System.out.print(cur.val + "->");
                stack.pop();
                pre = cur;
            } else {
                if (cur.right != null)
                    stack.push(cur.right);
                if (cur.left != null)
                    stack.push(cur.left);
            }
        }
    }

四、层次遍历

　　访问结果：GDMAFHZE。

    public void levelOrderTraverse(TreeNode root) {
        if (root == null) {
            return;
        }
        Queue<TreeNode> queue = new LinkedList<TreeNode>();
        queue.add(root);

        while (!queue.isEmpty()) {
            TreeNode node = queue.poll();
            System.out.print(node.val + "->");

            if (node.left != null) {
                queue.add(node.left);
            }
            if (node.right != null) {
                queue.add(node.right);
            }
        }
    }